1. Filed of the Invention
This invention relates to a mold for vulcanization building of a tire. More particularly, it relates to a mold suitable for the vulcanization building of the tire introducing so-called pitch variation into a tread pattern for reducing pattern noise during the running of the tire.
2. Description of Related Art
Since pattern noise resulting from the tread pattern largely occupies various tire noises, it is common to form a pitch variation in the tread pattern for reducing such pattern noise. In this case, the tread pattern is comprised of elements having a telescopic shape in only a circumferential direction of the tread, wherein a circumferential length ratio between the elements is varied by changing circumferential lengths of these elements and designing an arrangement of these elements, whereby a frequency characteristic of the pattern noise is flattened to reduce the noise.
The tire having such a pitch variation can be manufactured with a vulcanization mold wherein a portion of the mold contributing to the formation of the tread pattern is comprised of many segments of plural types having different circumferential lengths partitioned by partition lines in a widthwise direction of the mold, each of which lengths corresponds to each element constituting the tread pattern. In this case, it is required to combine the segments of the same types as the number of pitches formed in the pitch variation. At the present, it is generalized to arrange the elements of 4 or more types randomly or periodically as the tread pattern, so that it is obliged to increase the number of segment types used in the mold.
In the manufacture of these mold segments, an enormous cost and considerable labor are required, so that there is caused a problem contrary to the above. Particularly, there is caused a problem in the vulcanization mold for the manufacture of tires having grooves (lug grooves) extending across an equatorial plane of the tire or so-called rib-lug or lug type tread pattern.
That is, when a tread pattern is formed by defining land portions by at least one circumferential groove extending along a circumference of a tread and both tread ends and arranging many lug grooves extending across the circumferential groove in the land portion in the circumferential direction of the tread, the mold is required to have a structure that segments are partitioned at positions excluding protrusions for the formation of the lug grooves. Because, when a partition line between adjoining segments is existent on the protrusion, the outgassing and rubber flow are created at this partition line during the vulcanization and the formation of a groove bottom is not carried out by the protrusion as is expected. Hence there are caused the lowering of cut resistance and resistance to groove crack due to the decrease of rubber gauge at the lug groove bottom, occurrence of cracks from a partition position of the segment at the lug groove bottom and chipping of blocks accompanied therewith. Also, the formation of the protrusion extending between adjoining segments requires a higher technique for the manufacture of the mold, which undesirably increases cost.
Therefore, the protrusion for the formation of the lug groove is required to locate in each of the segments. However, if it is intended to form the lug type or rib-lug type tread pattern by the mold having such segments, there is a problem that the resulting tire does not develop the effect of the given pitch variation as is designed.
In the tread pattern having lug grooves, the most effective means for reducing the pattern noise is a change of interval between adjoining mutual lug grooves arranged on the same circumference from each other, and a pitch variation is designed based on this technical concept. In fact, the pitch variation is formed by combining segments having different circumferential lengths. However, such a pitch variation is not sufficiently reflected to the interval between the lug grooves because the lug groove is not formed in correspondence with the partition line between the segments as mentioned above and hence the circumferential length of the segment itself does not correspond to the interval between the lug grooves. Particularly, when the pitch variation is formed by combining segments types having a small difference in circumferential length between mutual segments, the interval between the lug grooves decreases from the circumferential length of the segment and hence the sufficient pitch variation is not formed. As a result, the interval between the lug grooves is largely remote from the designed pitch variation and hence the expected reduction of pattern noise is not attained.